Recombinant plasminogen activator inhibitor-1 protects platelets against the inhibitory effects of plasmin.

Plasmin-induced degradation of platelet glycoprotein Ib (GPIb), the von Willebrand factor (vWF) receptor, has been implicated as a mechanism contributing to the development of platelet dysfunction following cardiopulmonary bypass (CPB). The goal of this study was to assess whether biologically active recombinant plasminogen activator inhibitor-1 (rPAI-1), could antagonize the inhibitory effects of plasmin on GPIb. GPIb function, as evaluated by measuring vWF-dependent, ristocetin-induced platelet agglutination in human platelet rich plasma (PRP) was significantly impaired following incubation with plasmin (60 +/- 14% inhibition, p < 0.01). Inclusion of rPAI-1 (10 micrograms/ml) in the PRP antagonized this plasmin effect, restoring agglutination to 92 +/- 8% of the control value (p < 0.01). The effect of rPAI-1 on the enzymatic activity of plasmin was further evaluated in an amidolytic assay with the plasmin substrate S2251 where an apparent second order rate constant of plasmin inhibition by rPAI-1 of 9.4 x 10(4) M-1 S-1 was determined. Our results suggest that rPAI-1, by inhibiting both tissue plasminogen activator-induced plasmin generation and plasmin activity directly, may have clinical value for improving platelet function during and after CPB.